Allow adding generic attributes

Hi,

Thanks for your contribution.

Please provide a MR instead of a patch.

milos.kroulik → made their first commit to this issue’s fork.

So, I've created a MR with the code from #2. There's at least one issue for my use case - I would like to specify Masonry options in a data-attribute, which can contain a JSON like this: https://masonry.desandro.com/#initialize-in-html

So I removed the CSS identifier validation for now - perhaps there should be a way to specify expected value of the attribute in the definition?